Evolution of Galaxy Types and H i Gas Contents in Galaxy Groups

Abstract

Using the group crossing time t c as an age indicator for galaxy groups, we have investigated the correlation between t c and the group spiral fraction, as well as between t c and the neutral hydrogen gas fraction of galaxy groups. Our galaxy group sample is selected from the SDSS DR7 catalog, and the group spiral fraction is derived from the Galaxy Zoo morphological data set. We found that the group spiral galaxy fraction is correlated with the group crossing time. We further cross-matched the latest released ALFALFA 70% H i source catalog with the SDSS group catalog and have identified 172 groups from the SDSS survey whose total H i mass can be derived by summing up the H i mass of all the H i sources within the group radius. For the galaxies not detected in ALFALFA, we estimate their H i masses based on the galaxies’ optical colors and magnitudes. Our sample groups contain more than eight member galaxies, they cover a wide range of halo masses and are distributed in different cosmic environments. We derived the group H i mass fraction, which is the ratio of group H i mass to the group virial mass. We found a correlation between the H i mass fraction and the group crossing time. Our results suggest that long timescale mechanisms such as starvation seem to play a more important role than short timescale processes like stripping in depleting H i gas in the SDSS galaxy groups.

A young galaxy cluster in the old Universe

“…8c), σ P = 510.0 km s −1 , and the projected virial radius, R P = 0.64 Mpc. The Dressler-Shectman (DS) test (Dressler & Shectman 1988;Ai & Zhu 2018) implies that the cluster is already dynamically virialized (∆/N member <1.0, see Section 3 for details). The estimated halo mass is consistent with that from the empirical relationship between the velocity dispersion and halo mass calibrated by a weak lensing mass (Hamana et al 2009) (2.5×10 14 M ).…”

“…We applied the Dressler-Shectman (DS) test (Dressler & Shectman 1988;Ai & Zhu 2018) to the blue cluster (see Section 4) to check if the cluster has sub-structure. Clearly relaxed systems will have less sub-structures.…”

“…Campusano et al (2018) concluded that among 207 identified clusters at z = 0.04 − 0.09, 63% are dominated by early-type galaxies and 37% are dominated by late-type galaxies. Recently Ai & Zhu (2018) investigated spiral fractions of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7; Abazajian et al 2009) local clusters identified by Berlind et al (2006). They used a visual classification by the Galaxy Zoo 1 (Lintott et al 2011) to identify spiral galaxies.…”

“…Therefore, direct comparison of blue fractions between different approaches and also comparison with galaxy evolution models are not straightforward. In addition, the previous studies include as many clusters as possible, resulting in a mixture of diverse cluster properties controlling the blue fraction such as density environment, halo mass, and dynamical stage (e.g., Butcher & Oemler 1984;Ai & Zhu 2018). The variety of definitions of the blue fraction and the mixture of diverse cluster properties have prevented us from correctly understanding how unusual blue clusters are in the local Universe.…”

Extragalactic HI spectra data processing method based onStacking

“…谱线叠加 ( 2 样本选取 在做谱线叠加之前首先需要准确测量星系群中 每个成员星系中心的红移, 以保证在叠加时HI的中 心是对齐的, 这样才能准确获得每个星系的HI含量. 本文中的样本选自SDSS观测的Mr18数据 [11,12] , 从 成 员 星 表(http://lss.phy.vanderbilt.edu/groups/dr7/)中 可以找到这些星系群的每个成员的星表数据, 见 表 1. 星表中提供了样本星系群每个成员的赤经、 赤纬、 红移等信息, 图 1显示了这些星系群的红移 速度区间, 横坐标的范围是7200-12800 km/s, 该范 围是ALFALFA巡天数据的第3个(共4个)通道范围, 也是HI谱线最可能存在的速度区间; 纵坐标代表叠 加星系群的序号, 颜色条内的数字代表星系群的 成员数量, 从下至上与表 2中左侧的数据一一对应.…”